Okay, I'm no expert on multiplayer commands either, but hopefully I can be of some help:
Basically, 'SEND NET MESSAGE STRING playernumber,string$' will send a string accross the network to the specified player [calling 'SEND NET MESSAGE STRING 0,string$' will send the message to all players except for the computer the program is running on], while 'get net message' and 'net message string$()' will return the next message in the message que (to check if this exists, call 'if net message exist()=1')
Here's a basic loop to get all messages sent from other players:
get net message
while net message exists()=1
tempstr$=net message string$()
Print tempstr$
get net message
endwhile
Remember that when *you* send a message (the user clicks the send button), you need do two things:
1. Send that message using 'SEND NET MESSAGE STRING 0,string$'
2. Print that message, since your own message will not get sent to yourself.
Hope that helps,
-Xol